Why would I think that? I mean, obviously if the parents aren't taking care of their child properly, like those bozos who for some stupid reason tried to give their newborn infant soy milk rather than breast milk, the child is in danger. However, a vegan diet is just fine for people of all ages when balanced properly.
In fact, a well-balanced vegan diet is better than most kids' diets these days. Do you really think a diet of highly processed foods is better than one based in whole foods? That eating MacDonald's once or twice a week, pizza, and a ton of junk instead of lots of fruits and veggies is somehow healthful? There's a huge obesity epidemic in kids these days and their parents aren't considered neglectful, even though they are harming their children.
Bottom line: parents need to feed their kids right, whether they choose to include meat, dairy, eggs, or none of the above in their child's diet.